A new study with cancer-sniffing Beagles has given scientists more reasons to develop cancer screening methods using dogs' sense of smell.
The Beagles in the study successfully detected lung cancer 97% of the time, suggesting that dogs might be able to outperform medical tests.
Now scientists are looking at how they can incorporate dogs' superior sense of smell into cancer screening diagnostics. And they might have figured out a way.
How can dogs smell cancer?
Our bodies release odour substances in our sweat, breath and other bodily fluids. When we are ill, and the function of our cells is altered, the odour substances change, causing our bodies to produce new smells.
Many diseases have particular smells that can be detected by everyone – for example a distinct symptom of diabetic ketoacidosis is a fruity smelling breath and body odour. Cancer and tumour cells also release distinctive smells, but it would take a much stronger scent reception to identify. The reason why dogs can smell cancer and humans can't is because dogs have 300 million scent receptors while humans have only 5 million.
Dogs can detect substances diluted to parts per trillion - that’s the equivalent of one drop of blood in two Olympic-sized swimming pools. This means dogs are highly sensitive to the smells of substances of very low concentrations such as early-stage cancer cells.
According to CEO and co-founder of Milton Keynes charity Medical Detection Dogs, Dr Claire Guest, cancer’s odour substances show up in the body as soon as cancer cells start to develop and dogs can smell them. That means cancer can be detected at a far earlier stage than a lot of traditional diagnostics currently allow.
How accurate are cancer-sniffing dogs?
Some cancers are difficult to catch by current diagnostic methods. For example, increased levels of one of the very few biomarkers of prostate cancer PSA does not always mean men have prostate cancer. But when a PSA test indicates higher than normal levels, healthy men are often subjected to unnecessary invasive procedures that carry their own risks. Sometimes men with normal levels of PSA have prostate cancer but it gets missed because they don’t get referred for further investigation.
During one 2014 study, two dogs were presented with 900 urine samples and successfully identified those coming from men with prostate cancer 98% of the time. They had 16 false positives and four false negatives, which means they outperformed PSA tests - that's a false rate of 2%.
"The current blood test PSA has a 75% false indication rate compared to the dogs' published false positive rate of below 10%," told us the UK charity Medical Detection Dogs.
Lung cancer is also difficult to detect in its early stages with current imaging techniques such as X-rays and CT scans. One study showed X-rays have a 23% false negative rate, for example. The Beagles in the latest cancer-sniffing dogs study had a 97% success rate at identifying blood serum samples from people with lung cancer.
Many involved in the studies are hoping to be able to create a protocol where samples from hospitals are sent to cancer-sniffing dog training centres like the Medical Detection Dogs for inspection. But not everyone agrees.
Can dogs find a place in the doctor’s office?
Not all experts are convinced that dogs can assist physicians in diagnostics.
"It wouldn't be a realistic way to screen patients," says Dr Hilary Brodie, a professor in the Department of Otolaryngology at the University of California, “it would take an immense amount of resources to train dogs to recognize the many types of cancer that can affect humans.”
Dr Brodie also points out that while traditional tests are not perfect, doctors know their exact false positive and false negative rates. It would be hard to tell, even from a group of dogs that were trained together and returned a fixed success rate number, what rate of false positives or false negatives a dog would return. This is because a lot like humans, dogs can have off days when they under-perform or get bored, especially when the task is not very physically engaging as sniffing samples in a lab is.
That said, dogs’ ability to smell cancer can help scientists create screening products similar to pregnancy tests and glucometers.
How medicine can use cancer-sniffing dogs to improve tests
Finding out exactly what it is that dogs smell when they detect cancer is the next step in the research; and it could lead to the development of an electronic device that can detect cancer just as well as dogs can.
This is exactly what Dr Quinn, who did the study with the lung cancer sniffing Beagles intends to do next.
This could be done by removing the different biochemicals from body fluid samples from cancer patients one by one and offering them to dogs until they stop reacting, indicating that the cancer odour chemical is no longer present in the sample.
That's how the specific biomarkers for different cancers can be identified and used to manufacture cancer-screening methods that mimic dogs’ powerful sense of smell.
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Cancer-detecting dogs in an NHS trial
Milton Keynes charity Medical Detection Dogs trains dogs to detect the odour of human disease. It has partnered with the NHS and is carrying out an ethically approved study into dogs’ ability to detect urological cancers using their sense of smell and a proof-of-principle trial exploring the ability of dogs to detect breast cancer.
"Our dogs are trained to discriminate cancer against controls in samples from patients. The work is currently proof-of-principle and so far has shown excellent results. Our long term aim is for our work to play a significant role in the early diagnosis of disease everywhere, including the NHS," told us the charity.
The Medical Detection Dogs also assists scientists with research into the development of e-noses.
Can dogs sense cancer in their owners?
Although all dogs have a very strong sense of smell, it is unlikely that a dog that hasn't been trained would be able to detect cancer with accuracy.
In addition, some breeds might have a stronger sense of smell than others. There is no list of the best cancer-sniffing dog breeds but it is safe to assume that hunting breeds, like the Beagle, would be better at smelling cancer than others. Still, training would be required for reliable results.
That said, the first recorded case of a dog detecting cancer in a human, reported by the British medical journal The Lancet in 1989, describes a woman who had a dark spot on her leg that a doctor diagnosed as a mole. Her dog, however, seemed unusually disturbed by the mole, constantly sniffing and scratching it and even attempting to bite it off once. This peculiar behaviour caused the woman to seek a second opinion; and upon further investigation she was diagnosed with malignant melanoma.
A similar case occurred in 2001, leading to the first official study of dogs’ cancer-sniffing abilities.
Dr Claire Guest of the Medical Detection Dogs charity went to see a doctor after her dog began prodding an area in her chest. Tests revealed two tumours in her breast.
Although these anecdotal accounts prompted scientific research, it is very unlikely that pet dogs would be able to detect cancer in their owners without having had the appropriate training first.
All the dogs that took part in studies and clinical trials and returned high success rates were first trained for a minimum of 6 months to recognise the smell of bodily fluids from patients with the disease.
However, if scientific research keeps returning successful results, everyone, including people without dogs, would be able to benefit from powerful diagnostics technology inspired by man's best friend's incredible sense of smell.
If you are interested in the scientific literature on canine scent reception look at this paper on canine scent sensitivity published in the journal Applied Animal Behaviour Science.
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